This invention relates to the first stage of two-stage electrostatic precipitators having corona electrodes and non-corona electrodes, wherein particulates entrained in a gas are electrostatically charged in order to enable precipitation thereof from the gas by passing a stream of the gas having the particulates contained therein between the corona electrodes and non-corona electrodes of the first stage of the electrostatic precipitator.
The performance of electrostatic precipitators is often hampered by a phenomenon known as back corona. During operation of electrostatic precipitators a corona is created in the vicinity of the corona electrodes between the corona electrodes and the collecting electrodes. The ions formed in the corona attach themselves to particulates so as to cause these particulates to acquire electric charge, and the thus-charged particulates (together with the unattached ions from the corona) move as a current toward the collecting electrodes where they rest, forming a dust layer. As the dust layer forms, the current must continue its existence through the dust layer in order to reach the collecting electrode underneath the dust layer. Back corona sets in when the current density of the current through the dust layer on the collecting electrode exceeds a certain value. At this critical value the electric field through the dust layer produces electrical discharges in the dust layer and injects charges of polarity, opposite to that of the moving charges in the aforesaid current, into the gas stream and neutralizes at least partially the charge on the particles.
It is also a well established fact that the current through the dust layer is primarily due to ion current and only a very small portion is due to charged particles. See, for example: H. J. White, "The Role of Corona Discharge in the Electrical Precipitation Process", Electrical Engineering, January (1952) pg. 67-73. In other words, back corona is caused by current which has crossed the gap between corona wires and collecting electrode without having performed a major useful function. If one were to make the ions oscillate back and forth many times inside the gas stream before being collected, one would give the ions a better chance to attach themselves to particles and they would also contribute less to the current flowing through the dust layer.
Prior art proposals for apparatus for carrying out a two stage precipitation process, where the particles are charged in a first stage and collected in a second stage, are disclosed for example, in U.S. Pat. No. 2,440,455 to White and U.S. Pat. No. 2,251,451 to Heinrich. The collecting stage can be of the standard wire-plate arrangement having at least one wire as a corona electrode and at least one plate as a collecting electrode; a dc voltage is applied between the corona electrode and the collecting electrode and, if desired, pulse voltages may be superimposed on the dc voltage. Preferably an arrangement would be chosen which produces a field within the duct of enhanced uniformity.
White proposes in his said patent the use of a special 3-electrode arrangement. Ac is applied to non-discharge electrodes and pulses are applied to special discharge electrodes between the non-discharge ac electrodes.
Heinrich in his said patent proposes the use of asymmetrical current impulses, which makes the ac frequency equal to the pulse repetition frequency.
Judging from FIG. 2 of Heinrich's said patent and the apparatus described therein, it appears that he is thinking of pulses with a pulse width on the order of milliseconds. (Even though FIG. 2 shows no reference to time, Heinrich mentions on page 4, column 1, line 21 a desired impulse frequency of 100 pulses per second. Using this frequency and assuming that FIG. 2 is to scale indicates a pulse width in excess of 2 milliseconds.)